Load-balancing cleanup.

The starting point of this patch was allowing a caller (i.e. the metacd_http) to use an iterator different than those specified in the NS configuration. In other words, to thinly wrap the API.

Some code reorganisation took place:
  - function reordering (to be grouped by puposes)
  - obsoletes removals (scored iterators ... meaningless since the score is the weight).
  - functions moved in others modules (checks on the storage class of services, moved into service_info)
  - code factorized (minor container handling, in metautils_containers)

Change-Id: I625fd9413e0fa01d68d085f93160c026b9744c0f